The present invention relates to a seal assembly with a built-in sensor, particularly for rolling bearings.
For protecting the rolling bodies of rolling bearings from external pollution, normally consisting of solid or liquid particles, seal assemblies are fitted between the bearing rings. These consist of one or more annular shields fitted integral with one of the bearing rings, and which provide for sliding seal action on the other bearing ring by means of respective elastomeric lips and/or for labyrinth sealing.
The rotation speed of the movable ring, and consequently of the shaft integral with it, is normally determined using a speed sensor comprising a Hall-effect or magnetoresistance sensor, and arranged facing a phonic wheel fitted directly on to the shaft.
The phonic wheel is made of ferromagnetic material, and presents a number of equally-spaced, magnetically nonuniform portions. Here and hereinafter, the term "magnetically nonuniform portions" is intended to mean element portions whose passage in a magnetic field results in a variation of the magnetic field at said portions, e.g. a number of solids and voids, or a number of projections formed on a surface facing the sensor. When the phonic wheel is rotated by the shaft, the alternating magnetically nonuniform portions moving past the sensor result in a variation in magnetic inductance, and in the generation by the magnetic sensor of a periodic signal. This is supplied to a central processing unit, which, on the basis of the frequency of the signal, provides for computing the number of revolutions per unit of time of the wheel and consequently of the shaft.
A major drawback of the above solution is that, for ensuring accurate speed detection, the phonic wheel and sensor must be mounted perfectly opposite each other, with a minimum amount of, as far as possible, constant tolerance.